A study of more than 10,000 young people found that e-cigarettes act as a gateway to the real thing, making those that use them 2.5 times more likely to start smoking cigarettes.

The e-liquid in e-cigarettes contains nicotine, and some teenagers showed signs of addiction within one months of starting to use the devices.

Cigarettes have fallen relatively out of favor with teenagers, but this research follows recent National Institute of Drug Abuse findings that vaping is increasingly popular with the age group, many of whom are not sure what exactly they are smoking.

The study authors found that all cigarette alternatives raised the odds that the teenagers would later try cigarettes too, suggesting that changes to policy are needed to discourage young people from taking up any for of smoking.

Popular e-cigarettes may increase the risk that teenagers will take up smoking real cigarettes, according to new research from the University of California, San Francisco

Other alternatives, including hookahs, non-cigarette combustible tobacco such as cigarillos or smokeless tobacco, trigger conventional smoking just as quickly.

The study of teenagers is the first time scientists have compared the trendy products simultaneously with subsequent cigarette use, and follows other recent research that has shown the direct health risks of e-cigarettes.

Studying the effect of e-cigarettes on the behavior of teenagers is crucial because, as the study points out, 90 percent of adults who smoke tried their first cigarette before they were 18.

At the outset of teen the study, none of the 12- to 17-year-old male and female participants had smoked a cigarette. The researchers followed up with them 12 months later.

Study author and University of California, San Francisco dentistry professor Dr Benjamin Chaffee said: 'We estimated that 'ever use' of e-cigarettes was associated with 2.53 times greater odds of subsequent cigarette use.'

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He added: 'Among youths who had never smoked a cigarette at baseline, adjusted odds of any cigarette use initiation were approximately double for ever users of e-cigarettes, hookah, non-cigarette combustible tobacco and smokeless tobacco compared with never users.

'Odds of past 30-day cigarette use at follow-up were also approximately double for ever users of e-cigarettes, hookah, non-cigarette combustible tobacco and smokeless tobacco compared with never users.'

Dr Chaffee, of the Center for Tobacco Control Research and Education at the University of California, San Francisco, added that the use of more than one product increased the risk even more.

He said non-cigarette tobacco use among teenagers increased between 2011 and 2015, as e-cigarettes and hookahs became increasingly popular.

According to last year's NIDA data, about one third of students aged 18 and under use e-cigarettes.

Meanwhile, the rate of decreases in youth smoking has slowed over the past decade.

Dr Chaffee warned: 'In light of these observed associations between non-cigarette tobacco use and future smoking, novel tobacco products have the potential to undermine public health gains in combating the smoking epidemic.'

The study published in JAMA Pediatrics was based on a national questionnaire of 10,384 US adolescents who reported never having smoked a conventional cigarette when first asked in 2013 or 2014 and were then surveyed again in 2014 or 2015.

Dr Chaffee and colleagues said it is important to understand whether non-cigarette tobacco use encourages conventional smoking.

He said: 'We report three central findings. First, youths who initiated tobacco use with non-cigarette products were more likely to have smoked cigarettes one year later than were youths who had never used tobacco.

'Second, the odds ratios were of similar magnitude across products and between ever use and former and current use, suggesting that any use of non-cigarette tobacco, whether former or current, is similarly associated with future smoking.

What parents NEED to know about e-cigarettes

How they work:

E-cigarettes use a mixture of flavored liquids and nicotine to create a vapor.

This vapor is then inhaled by the user similarly to how one would smoke a regular cigarette.

Are these devices safe?

Since these devices don't use traditional smoke, people are under the assumption that they are safe for you.

But the liquid in the e-cigarettes can contain harmful toxins and carcinogens including anti-freeze.

The nicotine in the e-cigarettes also had addictive components and can lead to other tobacco use. This can hinder brain development in teens.

Also, the devices can overheat and explode if defective.

The Food and Drug Administration does not certify e-cigarettes as a product to get over smoking regular cigarettes.

'Ever users of multiple tobacco products were more likely to initiate smoking than were ever users of a single product, and product specific associations with future smoking were essentially independent, suggesting the risk of progressing to conventional cigarette smoking is increased with use of multiple forms of non-cigarette tobacco.'

Dr Chaffee said a number of factors could explain the findings, including e-cigs and other non-tobacco products inducing nicotine dependence, symptoms of which have been reported by youths who use cigars and smokeless tobacco, as little as once a month.

Teenagers who use the products may also find conventional cigarettes to be more convenient and effective in satisfying nicotine cravings.

Meanwhile, a small study from the Medical University of South Carolina found that using the devices was, in fact linked to better chances of quitting cigarettes for adults.

Of the 68 smokers involved, the half that were given e-cigarettes cut back their traditional tobacco use and took up the new device readily, even without instruction to do so.

'Combustible cigarettes are the most harmful form of nicotine delivery. Alternative delivery of nicotine, through e-cigarettes, could significantly reduce harm and the risks of cancer and other diseases to smokers,' said that study's lead author, Dr John Carpenter.

The study's findings suggest that 'these products might promote switching. Anything that gets smokers off combustible cigarettes is a good thing,' he added.

But the same does not seem to hold true for young people who have not started or formed a habit of smoking.

He added: 'This study's findings provide evidence that despite their differences, disparate alternative cigarette products contribute to a similar process that leads to cigarette use initiation.

'In policy terms, the findings provide a rationale to treat alternative cigarette products as a group and potentially extend policies that work for one product to the others, such as a ban on flavoring.'

Other studies have suggested that the flavoring used in e-cigarette liquid may well be harmful in itself.

'Even if youths do not progress to smoking cigarettes, any tobacco use is harmful. The estimated health risks of non-cigarette tobacco products should include the additional health consequences of future cigarette use,' said Dr Chaffee.

Last year a study of more than 44,000 school children in Canada found those who used e-cigs were nearly twice as likely to go on to become regular tobacco smokers.

Recent NIDA research also found that teenagers are increasingly using marijuana, and vaping may make it more accessible to them, even if they are unsure what is loaded into the devices before they use them.

E-cigarettes contain a liquid form of nicotine that is heated into vapor to be inhaled, avoiding the harm caused by tobacco smoke.

Health experts agree that the devices are much safer than smoking tobacco - and the gadgets are thought to have helped 22,000 people quit smoking each year.

But these findings add to the building evidence that while they may pose fewer risks than traditional combustible tobacco, e-cigarettes are far from harmless.